The operation of internal combustion engines, regardless of their particular design, such as reciprocating engine, pistonless rotary engine or free-piston engine, occurs in repeated strokes in each of which certain processes are carried out, such as intake and compression of a fuel and air mixture, combustion, and discharging of the combusted fuel air mixture, or the like. The sounds generated hereby propagate through the engine on the one hand directly as solid-borne sound and on the other hand they exit along with the combustion gases through the exhaust system or exhaust line of the engine.
The sounds propagating through the internal combustion engine as solid-borne sound can generally be well insulated by suitable insulating materials in the engine compartment of a vehicle.
To reduce the acoustic emissions escaping with the exhaust gases, sound-absorbing devices are usually arranged in the exhaust duct. Such silencers can operate, for example, according to the absorption and/or reflection principle. So-called active silencing or sound cancellation systems are also known, which superimpose electroacoustically generated anti-noise pulse trains on the sonic pulse trains transported with the combustion gases. Descriptions of such active silencing systems, also known as anti-sound systems, will be found, for example, in the documents U.S. Pat. No. 4,177,874, U.S. Pat. No. 5,229,556, U.S. Pat. No. 5,233,137, U.S. Pat. No. 5,343,533, U.S. Pat. No. 5,336,856, U.S. Pat. No. 5,432,857, U.S. Pat. No. 5,600,106, U.S. Pat. No. 5,619,020, EP 0 373 188, EP 0 674 097, EP 0 755 045, EP 0 916 817, EP 1 055 804, EP 1 627 996, DE 197 51 596, DE 10 2006 042 224, DE 10 2008 018 085 and DE 10 2009 031 848.
However, for several reasons a complete elimination of exhaust sounds is not desirable. On the one hand, an almost silent vehicle represents a substantial safety risk in road traffic, since a traffic participant can only then recognize it, when it is already in his or her's central field of vision. A traffic participant will therefore normally not perceive extremely low-noise vehicles approaching from the side or even from behind. Furthermore, most vehicle drivers are used to estimating the speed and acceleration of their vehicle and potential irregularities in the vehicle's drive system by means of the exhaust sounds. Thus, for example, the noise reduction associated with cylinder cutoff when the vehicle is at standstill frequently causes concern among the passengers as to a possible malfunctioning of the vehicle's drive system. Finally, it should also be mentioned that the impression that a vehicle leaves on people is dictated not only by its optical appearance, but also to just as great a degree by the acoustic pattern of its driving noise and especially its exhaust sound.
In the case of modern Diesel vehicles and vehicles with hybrid drive systems it is generally no longer possible to judge the actual engine power or vehicle speed in the usual way from the exhaust sound. Just so, a driver of a vehicle with cylinder cutoff engaged can never be quite certain he has not stalled the engine.
Therefore, active sound systems have been developed for use in exhaust systems of vehicles with which it is possible to generate an exhaust sound synthetically. Corresponding systems have an electroacoustical transducer that is connected to the exhaust line of an internal combustion engine by a connector piece in order to superimpose electroacoustically generated sonic waves on the sonic waves stemming from the combustion process in the engine. In this way, the exhaust sounds of a vehicle can be deliberately modified. The electric input signal of the transducer is generated by a control as a so-called control signal, taking into account current values of engine parameters, such as engine speed or firing order.
Present embodiments of such control have a software processing device for generating the control signal, in which the particular control signal generated is produced according to the exhaust sound pattern desired for the particular engine operating state. Due to technical limitations, the frequency range of such a software-generated control signal is at present limited to around 500 Hz, however, with the consequence that the resulting exhaust sound is perceived as being synthetic and not natural. By a natural sounding exhaust sound is meant here an exhaust sound with an acoustic pattern as is created with traditional exhaust systems making use of mufflers.
For a natural appearing acoustic pattern, the control signal should have higher frequency components, yet generating these separately is a heavy burden on the control device and therefore not practical.